1. Field of the Invention
The invention relates to a process for the production of multi-layer coatings.
2. Description of the Prior Art
Automotive coatings consist, as a rule, of a separately baked electrodeposition coating (EDC) primer, a separately baked primer surfacer layer (filler layer) applied thereto and a top coat applied thereto comprising a wet-on-wet applied color- and/or special effect-imparting base coat layer and a protective, gloss-imparting clear coat layer. The total primer surfacer plus base coat layer thickness is generally 30 to 60 μm.
Processes are known from WO 97/47401 and U.S. Pat. No. 5,976,343 for the production of decorative multi-layer coatings, which processes allow for the elimination of the application and separate baking of a primer surfacer layer which, of course, reduces coating material consumption and the total layer thickness. These processes have in common the fact that a multi-layer coating structure comprising a first, modified water-borne base coat, a second, unmodified water-borne base coat and a clear coat is applied by a wet-on-wet-on-wet process comprising the joint curing of these three coating layers that are applied to a baked EDC primer. In practice, these processes use two base coat layers that allow for markedly lower total layer thicknesses by approximately 15 to 25 μm, than that of a conventional primer surfacer and base coat. The modified water-borne base coat is produced in these processes from an unmodified water-borne base coat by mixing with an admixture component and is intended to replace the function of a conventional primer surfacer. WO 97/47401 recommends as an admixture component, the addition of polyisocyanate crosslinking agent, while U.S. Pat. No. 5,976,343 describes the addition of polyurethane resin.
A weakness of the processes known from WO 97/47401 and U.S. Pat. No. 5,976,343 is that it is not readily possible to produce multi-layer coatings in certain color shades (“problematic color shades”). The reason is UV light (UV radiation), as a constituent of natural daylight, passes through the coating layers applied to the EDC primer to the surface of the EDC primer to a noticeable extent in the absence of a primer surfacer layer and causes degradation of the EDC primer.
The color shades which are problematic with regard to the production of primer surfacer-free multi-layer coatings are those which, while (like unproblematic color shades) providing a coating which appears to an observer to be opaque, permit an inadmissibly large amount of UV light to penetrate through the multi-layer structure consisting of clear coat, unmodified water-borne base coat and modified water-borne base coat to the surface of the EDC primer and cause long term damage to the EDC layer. Such problematic color shades are to be found both among single (plain) color shades and special effect color shades. Examples may, in particular, be found among water-borne base coats with dark blue single color shades based on phthalocyanine pigments and among water-borne base coats with specific special effect color shades, for example, dark blue metallic color shades or light metallic color shades, such as, in particular, silver color shades and among water-borne base coats with specific special effect color shades containing elevated proportions, for example, 50 wt. % or more, of mica pigments (special effect pigments on the basis of coated, in particular, metal oxide-coated mica) in the pigment content. In the case of the problematic color shades, the UV light may penetrate through the multi-layer coating structure, for example, to an extent exceeding the specified UV transmission level and reaches the EDC layer.
Car manufacturers' specifications state, for example, that UV transmission through the base coat layer in the area of the complete outer skin of the vehicle body should amount to less than 0.1% in the wavelength range of from 280 to 380 nm and less than 0.5% in the wavelength range of from 380 to 400 nm. The possible undesired long-term consequences of an inadmissible level of UV light penetration to the EDC layer are chalking of the EDC layer and delamination of the multi-layer coating over the service life of the coated substrates.
Alternatively, the modified and/or the unmodified water-borne base coat could be applied in an overall higher layer thickness sufficient to prevent to an adequate degree the access of UV light to the EDC primer. However, this would be a backward technological step in the direction of high total film thickness.
The use of UV absorbers in clear coats or base coats is known, for example, from U.S. Pat. No. 5,574,166 and WO 94/18278, and is a solution to the problem of delamination. However, UV absorbers cannot be used to a very great extent in the base coat layers and/or the clear coat layer because of the migration tendency of the UV absorbers and because of the gradual degradation of the UV absorbers, as well as for cost reasons.
Other solutions, which approach the delamination problem from the EDC side are known from EP 0 576 943 A1, U.S. Pat. No. 6,368,719, U.S. 2003/0054193 A1 and U.S. 2003/0098238 A1. These disclose the use of EDC coating compositions which are resistant to the action of UV light due to specially selected binders or due to the addition of suitable additives. This inevitably restricts the EDC composition, such that concessions may have to be made in relation to other technological properties, such as, for example, corrosion protection.
The addition of aqueous filler (extender) pastes containing polyurethane resin to water-borne base coats is known from U.S. Pat. No. 5,968,655. The filler pastes may contain pigments. The water-borne base coats modified by addition of the filler pastes are applied onto EDC-primed substrates, overcoated with unmodified water-borne base coat and clear coat and baked together. The above-mentioned problem solved by the present invention of excessively high UV transmission is neither directly nor indirectly addressed in U.S. Pat. No. 5,968,655.
U.S. Pat. No. 6,221,949 discloses a process for the production of a multi-layer coating wherein a three-layer coating consisting of an up to 35 μm thick coating layer, of a water-borne base coat layer and of a clear coat layer is applied onto an EDC primer and the three coating layers are jointly baked. The coating layer, which is up to 35 μm thick, is applied from an aqueous coating composition, which contains a water-dilutable polyurethane resin as a binder and pigments and/or fillers. With regard to the pigments, it is merely stated that talc has proven itself as a pigment or filler and its content in the total amount of pigments and fillers is from 20 to 80 wt. %. In the examples, talc and titanium dioxide are combined with barium sulfate, iron oxide pigments and/or perylene pigment.
U.S. Pat. No. 6,221,949 does not address, either directly or indirectly, the problems of excessively high UV transmission to the EDC primer. However, the problems of excessively high UV transmission may occur even in the case of the process according to U.S. Pat. No. 6,221,949, especially in the case of problematic color shades. If the proposals regarding composition of the pigment content that may be inferred from the example section of U.S. Pat. No. 6,221,949 are followed, although a multi-layer coating having sufficiently low UV transmission to the EDC primer may be obtained, the desired color shade may not be achieved, at least in the case of problematic color shades, if the base coat layer is applied in a low coating thickness, especially below its black/white opacity (black/white hiding power).
WO 2005/021168 refers, in the paragraph connecting pp. 12 and 13, to a further development of the process known from DE 44 38 504 A1 (the German equivalent of U.S. Pat. No. 6,221,949). In the following paragraph, it is said to be fundamental to the invention that the coating composition used in the process as the first base coat contains as a fundamental component at least one (co)polymer or graft copolymer that is produced in the presence of a polyurethane specified in greater detail.
The term “black/white opacity” is used in the description and the claims. It refers to the dry coating thickness of a coating composition wherein the contrast between the black and white fields of a black and white chart coated with the coating composition is no longer discernible. Following ISO 6504-3 (method B), in order to determine this coating thickness, the coating composition of which the black/white opacity is to be investigated may be applied in a wedge shape onto a black and white chart and dried or hardened.
It has been found that it is possible to produce multi-layer coatings with a low total coating thickness and in the desired color shade without separate baking of a conventional primer surfacer layer, and to be able to sufficiently prevent a long term damaging access of UV light to the EDC primer if a first thin coating layer of an aqueous coating composition that has been pigmented in a particular manner, a second coating layer of a water-borne base coat in a coating thickness below its black/white opacity, and a clear coat layer are applied wet-on-wet-on-wet and jointly baked.